Fix: python: -Wshadow warning

Fix: python: -Wshadow warning

Fixes warning when building Python Bindings with SWIG 3.0.12.

  lttng_wrap.c: In function ‘SWIG_Python_NewShadowInstance’:
  lttng_wrap.c:2483:65: warning: declaration of ‘swig_this’ shadows a global declaration [-Wshadow]
   SWIG_Python_NewShadowInstance(SwigPyClientData *data, PyObject *swig_this)
                                                                   ^~~~~~~~~
  lttng_wrap.c:2224:18: note: shadowed declaration is here
   static PyObject *swig_this = NULL;
                    ^~~~~~~~~
  lttng_wrap.c: In function ‘SWIG_Python_SetSwigThis’:
  lttng_wrap.c:2558:51: warning: declaration of ‘swig_this’ shadows a global declaration [-Wshadow]
   SWIG_Python_SetSwigThis(PyObject *inst, PyObject *swig_this)
                                                     ^~~~~~~~~
  lttng_wrap.c:2224:18: note: shadowed declaration is here
   static PyObject *swig_this = NULL;

Signed-off-by: Francis Deslauriers <francis.deslauriers@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Change-Id: Icde5f124e914a158eb190f95e3c012b7f800d7d1